1. Field of the Invention
This invention relates to an image recording medium on which an image can be recorded as a latent image and a method of manufacturing the image recording medium.
2. Description of the Related Art
In order to reduce irradiation dose to the patients and/or to improve diagnostic performance of the X-ray image in a medical radiography, there have been proposed various systems in which a photoconductive body sensitive to X-rays is used as an image recording medium, and an electrostatic latent image formed on the photoconductive body upon exposure to X-rays is read out. For example, see U.S. Pat. Nos. 4,176,275, 5,268,569, 5,354,982, and 4,535,468, “23027 Method and Device for recording and transducing an electromagnetic energy pattern”; Research Disclosure June 1983, Japanese Unexamined Patent Publication No. 9(1997)-5906, U.S. Pat. No. 4,961,209, and “X-ray imaging using amorphous selenium”; Med Phys. 22(12).
For example, the image recording medium disclosed in U.S. Pat. No. 4,535,468 comprises a conductive substrate (which functions as a recording light side electrode layer) which is formed of, for instance, a relatively thick (e.g., 2 mm) aluminum plate and is permeable to recording light (an electromagnetic wave), and a recording photoconductive layer which is formed of a photoconductive material containing a-Se (amorphous selenium) as a major component and is 100 to 500 μm in thickness, an intermediate layer (trapping layer) 0.01 to 10.0 μm thick which is formed of, for instance, AsS4, As2S3 and/or As2Se3 and in which an electric charge of a polarity of latent image generated in the recording photoconductive layer gets trapped and accumulates, a reading photoconductive layer which is formed of a photoconductive material containing a-Se as a major component and is 5 to 100 μm in thickness and a reading light side electrode layer which is formed of, for instance, Au or ITO (indium tin oxide) 100 nm thick and is permeable to reading light (an electromagnetic wave) which are formed on the conductive substrate in this order. There are further disclosed that it is preferred that the reading light side electrode layer be used as the positive electrode layer from the viewpoint of better use of mobility of positive holes and that deterioration in S/N ratio due to direct injection of an electric charge from the electrode layer can be prevented by providing a blocking layer of organic material between the reading light side electrode layer and the reading photoconductive layer. That is, the recording medium is a multi-layered recording medium which is formed of a plurality of layers of photoconductive material containing a-Se as a major component and is high in dark resistance and response speed to reading.
In order to increase the S/N ratio and to effect reading simultaneously at a plurality of places (normally arranged in the main scanning direction) to shorten the reading time, the reading light side electrode is sometimes shaped into a stripe electrode comprising a plurality of line electrodes arranged at intervals equal to the pixel pitch. See, for instance, Japanese Unexamined Patent Publication No. 10(1998)-232824. However it is difficult to form a stripe electrode layer on the reading photoconductive layer of the recording medium disclosed in the aforesaid U.S. Pat. No. 4,535,468. This is because the stripe electrode layer is formed by photo-etching a solid electrode layer and a-Se in the reading photoconductive layer deteriorates in its properties under a high temperature (e.g., 200° C.) to which the reading photoconductive layer is subjected during, for instance, baking photoresist.
Further, alkali developer used for developing the photoresist emits harmful gas when brought into contact with the photoresist, and removal of the harmful gas complicates the manufacturing procedure and adds to the cost.
This applicant has proposed, in Japanese Unexamined Patent Publication No. 10(1998)-232824, an image recording medium (an electrostatic recording medium) comprising a recording light side electrode layer which is formed of SnO2 (nesa film) and is permeable to recording light (radiation), a recording photoconductive layer which is formed of a photoconductive material containing a-Se as a major component and is 50 to 1000 μm in thickness, a charge transfer layer which is formed of, for instance, a-Se doped with 10 to 200 ppm of organic material or Cl and forms a charge accumulating portion for accumulating an electric charge of a polarity of latent image generated in the recording photoconductive layer on an interface between the recording photoconductive layer and the charge transfer layer, a reading photoconductive layer which is formed of a photoconductive material containing a-Se as a major component and a reading light side electrode layer which is permeable to reading light which are superposed one on another in this order.
In the specification of Japanese Unexamined Patent Publication No. 10(1998)-232824, there is no clear disclosure as for from which side the layers are formed, that is, whether the recording light side electrode layer is formed first and the reading light side electrode layer is formed last, or the reading light side electrode layer is formed first and the recording light side electrode layer is formed last. This means that the layers may be formed in whichever order. However, in the specification, there is proposed to use a conductive material layer such as a nesa film formed on a transparent glass plate (support) as the reading light side electrode layer and to use the reading light side electrode layer as the positive electrode layer. There is further proposed to form the reading light side electrode layer, by use of the semiconductor forming technique, as a stripe electrode layer or a comb electrode layer comprising a plurality of comb teeth electrodes arranged at intervals equal to the pixel pitch. In this case, the stripe electrode layer is first formed on a transparent glass substrate by photo-etching or the like and then the reading photoconductive layer to the recording light side electrode layer are formed on the reading light side electrode layer. Though not clearly shown in the specification, it is easy for a person with ordinary skill in the art to come up with the idea of setting the pixel pitch to 50 to 200 μm since it is important in the medical radiography to obtain a high S/N ratio with a high sharpness.
As in the aforesaid U.S. Pat. No. 4,535,468, we have proposed in the aforesaid Japanese Unexamined Patent Publication No. 10(1998)-232824 to prevent deterioration in S/N ratio due to direct injection of a positive electric charge on the reading light side electrode layer by providing a blocking layer about 500 Å thick of inorganic material such as CeO2 between the reading light side electrode layer and the reading photoconductive layer.
We have further studied the image recording medium proposed in our Japanese Unexamined Patent Publication No. 10(1998)-232824 and have found the following points.    1) A method of forming the stripe electrode layer in which a relatively thin (e.g., 50 to 200 nm) ITO film is first formed on a transparent glass substrate and the ITO film is shaped into a stripe electrode layer by photo-etching is suitable for forming a fine stripe pattern at low cost.    2) By forming the recording photoconductive layer of an a-Se layer 50 to 1000 μm thick, a higher dark resistance is obtained.    3) As the charge transfer layer, a laminated positive hole transfer layer, formed by a first positive hole transfer layer 0.1 to 1 μm thick which is of organic material and accumulates electrons to form a charge accumulating portion and a second positive hole transfer layer 5 to 30 μm thick which is formed of a-Se doped with 10 to 200 ppm of Cl, transfers positive holes at high speed and is less in positive hole traps, is advantageous from the viewpoint of afterimage and response speed to reading.    4) To form the reading photoconductive layer of an a-Se layer 0.05 to 0.5 μm thick is advantageous in obtaining a high dark resistance.    5) When the charge transfer layer is in the form of a laminated positive hole transfer layer comprising a first charge transfer layer 0.1 to 1 μm thick which is of PVK, TPD or the like and a second charge transfer layer 5 to 30 μm thick which is formed of a-Se doped with 10 to 200 ppm of Cl, the first charge transfer layer comes to exhibit high resistance to the electric charge of the latent image polarity (the polarity of latent image) while the second charge transfer layer comes to transfer the electric charge of the transfer polarity (the electric charge of the polarity to be transferred) at high speed, which is advantageous from the viewpoint of afterimage and response speed to reading. However, when the second charge transfer layer is replaced by an a-Se layer 5 to 30 μm thick and the a-Se layer is caused to double the second charge transfer layer and the reading photoconductive layer, a relatively excellent image recording medium can be manufactured with the manufacturing procedure simplified.
That is, the image recording medium proposed in our Japanese Unexamined Patent Publication No. 10(1998)-232824 is an excellent multi-layered recording medium which is high in dark resistance and response speed to reading, and is preferably formed of a plurality of layers of photoconductive material containing a-Se as a major component.
As is well known, in an a-Se film, crystallization progresses with time, which can give rise to a so-called bulk crystallization problem that especially the dark resistance deteriorates. The bulk crystallization significantly occurs when the a-Se film is of non-doped or pure a-Se and progresses at higher speed as the temperature is higher. Accordingly, the aforesaid image recording medium which comprises many layers of non-doped a-Se is severely limited in working temperature and service life.
Further, it has been well known that interfacial crystallization progresses on an interface between an a-Se film and another material during the step of depositing films. For example, when the recording light side electrode layer is deposited on the recoding photoconductive layer, the interfacial crystallization is apt to progress on the interface between the recording photoconductive layer and the recording light side electrode layer, which causes an electric charge to be directly injected into the recording photoconductive layer from the recording light side electrode layer during recording (where a high electric voltage is applied), which deteriorates the S/N ratio. When the electrode layer is of a transparent oxide film, especially an ITO film, the interfacial crystallization markedly progresses and deterioration in S/N ratio is significant.
In the image recording medium described above, a latent image is recorded by accumulating in the charge accumulating portion the electric charge of the latent image polarity generated in the recording photoconductive layer upon exposure to a recording electromagnetic wave passing through an object, and reading is carried out by coupling of charged pairs, generated in the reading photoconductive layer upon exposure to a reading electromagnetic wave passing through the reading light side electrode layer, with the electric charge of the latent image polarity in the charge accumulating portion.
The charged pair generating efficiency of the recording photoconductive layer is proportional to the strength of the electric field formed between the charge accumulating portion and the reading light side electrode layer. When the amount of the recording electromagnetic wave is reduced in order to reduce irradiation dose to the patients, the charge of the latent image polarity accumulated in the charge accumulating portion is reduced and the electric field formed between the charge accumulating portion and the reading light side electrode layer becomes weak, which results in poor charged pair generating efficiency and deterioration in sensitivity of the image recording medium to the reading light. Increase of the amount of reading light in order to compensate for deterioration in sensitivity of the image recording medium to the reading light gives rise to a problem of increase in the cost or the like.